1. Field of the Invention
The invention relates to lasers, in particular mode-locked lasers.
2. Description of the Related Art
Passive mode-locking is a known method for generating with lasers ultrashort pulses (i.e. pulses of less than 100 ps FWHM) (see e.g. N.R. Belashenkov et al.: YAG: Nd.sup.3 +LASER FOR STUDY OF HIGHSPEED PROCESSES, Instr. +Exp. Techn. No. 1, 1987, 204-205). For passive mode-locking it is known to arrange a thin cuvette with a dye solution absorbing the laser wavelength just in front of a resonator mirror in the resonator of the laser. Since with suitably selected dyes the transmission of such a dye solution depends on the incident light intensity because higher light intensities set more dye molecules into an excited state in which they cannot absorb the laser light or can only slightly absorb the laser light, i.e. are bleached, on oscillation buildup of the laser when pronounced fluctuations (photon noise) are still present only the respective most pronounced noise peak is allowed to pass through the dye solution with relatively little attenuation whilst the weaker noise peaks are highly suppressed. In the resonator a pulse running to and fro and becoming increasingly steeper and shorter is formed and each time said pulse strikes the coupling-out mirror a certain fraction thereof is coupled out so that in this manner at the output of the laser a train of ultrashort pulses appears.
The disadvantages of this method reside in particular in the low photochemical stability of the dye solution and in the critical setting of the dye concentration and of the position and thickness of the dye cuvette. The frequently required emission of the half wavelength can be produced in this case only by external frequency doubling in a frequencydoubling crystal and not by the generally particularly efficient doubling within the resonator (see e.g. Falk et al.: Pulse Width of the Mode-Locked, Intervally Frequency-Doubled Laser, IEEE Journal of Quantum Electronics, Vol. QE-11, No. 7, July 1975, 365-367; S. Umegaki: An Efficient Method of Second Harmonic Generation Interval to Laser Cavity, JAPAN. J. Appl. phys. Vol. 15 (1976) No. 8, 1595-1596) since the dye solutions at half the laser wavelength frequently exhibit pronounced absorption which leads to high losses.
In addition, for many uses two different types of lasers are required which furnish exactly time-synchronized trains of ultrashort laser radiation pulses. Hitherto this could only be done with considerable expenditure. In a known method for synchronization of two lasers a mode-locked master oscillator is used which synchronously pumps another laser (normally a dye laser) either directly or with its second harmonic. A disadvantage here is that the master oscillator itself must be mode-locked and that the number of combinations of different lasers which can be synchronized in this manner is limited.
In another known synchronization method, in a dye laser a laserable absorber dye is used so that a mode locking of the two lasering media occurs (Z. A. Yasa et al., Appl. Phys. Lett. 30, No. 1, 1977, 24-26; see also W. Dietel et al., OPTICS COMMUNICATIONS, 35, No. 3, Dec. 1980, 445, 446). The disadvantage of this method resides in that the pump laser wavelength must be adapted to the absorption of the laser dye and this again limits the number of possible combinations. It is also frequently desired to generate ultrashort pulses with different wavelengths, for example with the sum or difference frequency of the radiation of two mode-locked lasers. Hitherto, for this purpose generally an additional nonlinear medium was used.